Electrical cables used in housing and industrial projects typically include an electrical conductor surrounded by at least one additional layer of material. In some cases, an insulating layer of material is used to insulate the conductor. The insulating layer is then surrounded by a layer of thermoplastic material, and this outermost layer may be referred to as a “sheath” or a “jacket.” Installation of electrical cable requires the cable to be threaded or passed through sections of a building, such as walls, ceilings, ducts and other conduits.
The most common electrical cable used in housing and industrial projects in the United States is called THHN (“Thermoplastic High Heat-resistant Nylon coated”). A typical THHN cable uses copper as an electrical conductor, polyvinyl chloride as the insulating material and nylon as the sheath material.
It has long been known to provide a lubricant on the sheath of the cable in order to reduce the coefficient of friction of the cable and make the cable easier to pull through conduit and other building structures during installation. Such methods have included manually applying a lubricant to the sheath just prior to installation, adding a separate lubricating layer to the sheath, and, most preferably, incorporating the lubricant into the sheath prior to forming the sheath.
The sheath layer is typically formed over the conductor core and insulating layer by an extrusion method. A lubricant can be incorporated directly into the cable sheath prior to extrusion by several methods, including but not limited to:
a) adding the lubricant to the sheath material and allowing the lubricant and sheath material to mix during the extrusion process;
b) pre-mixing the lubricant with the sheath material prior to adding the sheath material to the extruder; and
c) pre-forming a highly concentrated lubricant composition (i.e., a masterbatch) and adding this composition to the sheath material in the extruder hopper.
For cost and other considerations, it is preferable to utilize a masterbatch composition to form the lubricated cable sheath. Silicone-based masterbatch compositions are described in, e.g., U.S. Pat. Nos. 7,410,695, 6,080,489, 5,708,084 and 5,391,594. Commercial masterbatches made of silicon rubber dispersed in a number of carrier resins, including nylon, are well known in the art.
Masterbatch compositions are formed by a melt mixing process, in which the masterbatch components are combined in a mixer, heated and blended. Once the temperature required to ensure sufficient blending of the components is reached the mix is removed from the mixer, cooled, and diced or pelletized.
A masterbatch composition must contain a base material that is compatible with the material into which the masterbatch composition will be added—if they are not compatible the masterbatch composition will not mix well with the base material and cannot easily be incorporated into the material. Thus, masterbatch compositions designed for incorporation into a nylon product (e.g., the nylon jacket of a THHN cable) have traditionally contained nylon as a base material because of incompatibility issues between the nylon product and other known masterbatch base materials such as polyethylene.
It is particularly difficult to make a masterbatch composition containing nylon and a silicon elastomer, however, because of volatility problems. The melt mixing process for these components requires temperatures of approximately 230-250° C. At these temperatures, commercial silicon elastomer materials such as methyl vinyl silicon rubber (“VMQ”)—as classified according to ASTM D-1418—produce volatile materials that vaporize during the melt mixing process, resulting in excessive porosity of the masterbatch composition. It would thus be preferable to avoid using nylon as a base material in a silicon elastomer masterbatch composition.
Polyethylene-based silicon elastomer masterbatches are known and can be made in a melt mixing process at temperatures of about 130-150° C. These lower processing temperatures cure the excessive porosity problems inherent in nylon-based silicon elastomer masterbatch compositions. Polyethylene-based silicon elastomer masterbatches have previously been found to be unsuitable for use in nylon-based products, however, because of the incompatibility problems discussed above.
It would thus be desirable to form a lubricated thermoplastic article from a masterbatch composition containing a silicon elastomer that does not suffer from excessive porosity problems caused by volatilization of silicon elastomer components during the manufacture of the masterbatch composition. More specifically, it would be desirable to form a lubricated nylon sheath for a THHN electrical cable using a silicon-based masterbatch composition that (1) does not have the excessive porosity problems of nylon-based silicon masterbatches and (2) is compatible with the nylon base material.